Furthermore most drill/drivers define a central axis along which the output shaft and the attached bit lie and, hence, rotate.
Should the body of the drill/driver be bulky, however, it is possible for some areas of potential use to become inaccessible. For example, a large battery for powering the drill/driver, or a large integral gearbox could make its outer dimensions so large that they extend far beyond the lateral sides of the drill/driver output shaft or its attached bit. This could mean, therefore, that the operator of the drill/driver is unable to use it in enclosed spaces (or next to walls) as the sheer bulk is too great to permit alignment of the bit with the work piece it is intended for the bit to act upon.
The possibility, therefore, of being able to axially offset the output shaft of a drill/driver relative to the central output axis is an attractive proposition, as this allows greater accessibility to confined areas, particularly with a hexagonal bit drive output spindle of relatively small diameter compared to the overall outside dimensions of the drill/driver. If, for example, the drill/driver were to utilise a relatively large chuck (large as compared to the overall outside dimensions of the drill/driver) then little advantage would be gained from being able to axially offset the position of the chuck, or its drive shaft. Such proposals are known and one example is that of a screwdriver sold by the Japanese tool manufacturer, National, under model number EZ 7680. This screwdriver has an output shaft which is permanently axially offset from the central motor output shaft, yet is freely rotatable in a plurality of positions thereabout. The output shaft may be locked in any one of these plurality of positions.
DE-A-3834886 discloses a hand-held electric drill having a gearbox 7 with the tool spindle 8 laterally offset with respect to the motor 2 armature shaft 3 and coupled therewith via gearing. The gearbox housing 10 is mounted to the front end of the motor housing 1 so that it can be angularly adjusted or rotated and is held in a fixed position by means of a connection ring 11 which ma be a clamp and can comprise indexing means.
However, in certain circumstances an operator of a drill/driver may wish to lock the output shaft against rotation thereby to facilitate manual operation of a screwdriver bit or a drill bit. This need may arise where delicate or controlled drilling or screwdriver work is required such as in fragile work pieces. Using a drill/driver such as the National toot described above in these circumstances, however, is not ideal. The fact that the output shaft is radially offset from the central drive shaft of the drill/driver means that very often a uniform application of torque in a manual mode is not possible. Even if it is possible, however, because the axis of application of applied force is not central with the axis of application of output drive, then problems such as slipping of the screwdriver bit from the screw head due to misalignment, for example, can occur.
A drill/driver which combines the facility to radially offset the rotational shaft driving the screwdriver bit or drill bit with the facility to allow the rotational drive shaft to be coaxial or collinear with the shaft along which the force is applied for use in a manual mode is a desirable aim.